Abstract

Forty-four river, stream, and drain sites in the Upper Colorado River
Basin in Colorado were sampled during 199698 to determine the occurrence
and distribution of pesticides in the basin. In a fixed-station study,
57 surface-water samples were collected from October 1996 through January
1998 at four sites. Each site was sampled approximately monthly for up
to a year, with more frequent sampling during the spring and summer growing
season. In a synoptic study, surface-water samples were collected at 43
sites in the agricultural areas of the Grand Valley and the Uncompahgre
River Valley in May 1998. Each site was sampled once.

Pesticide concentrations generally were low and varied seasonally and
across land uses. Thirty-five pesticides were detected at least once in
the two studies, and 11 pesticides accounted for almost 82 percent of
the detections. Herbicides were more commonly detected than insecticides,
and only the herbicides alachlor and atrazine were detected in more than
50 percent of the samples. Carbofuran was the most commonly detected insecticide
and was detected in 19 percent of the samples. Pesticide detections increased
and were measured at higher concentrations in the summer months and at
the agriculture sites. All pesticide concentrations were less than drinking-water
standards, and most complied with human-health advisories and criteria
for the protection of freshwater aquatic life. Pesticides in the Upper
Colorado River Basin in Colorado were detected slightly less frequently
and generally at lower concentrations than in 20 National Water-Quality
Assessment Program study-unit basins that collected water-quality data
from 1992 through 1996. Results from surface-water sampling conducted
during 199698 in the Upper Colorado River Basin in Colorado indicate
that beneficial uses of water were not being impaired by the presence
of pesticides in surface waters in the basin.

Table of Contents

Foreword

Abstract

Introduction

Purpose and Scope

Description of the Upper Colorado River
Basin Study Unit

Study Design

Sampling and Laboratory Methods

Acknowledgments

Quality-Control Methods and Results

Hydrologic Conditions During Sampling

Occurrence and Distribution of Pesticides

Pesticides at Fixed-Station Network

Temporal Distribution

Spatial Distribution

Pesticides at Synoptic Network

Comparison of Pesticides at Fixed-Station
Network and Synoptic Network